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From "Ard Schrijvers (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JCR-1213) UUIDDocId cache does not work properly because of weakReferences in combination with new instance for combined indexreader
Date Wed, 14 Nov 2007 12:24:43 GMT

    [ https://issues.apache.org/jira/browse/JCR-1213?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12542432
] 

Ard Schrijvers commented on JCR-1213:
-------------------------------------

"I guess we just have a differing judgment about when and how often the cost of building cache
entries and maintaining them should be spent." 

I think we have the same end goal: If the index segment instance in which the parent lookup
was found is still valid --> return cached docNumber. This is the main performance we look
for. And I also think we agree on the implementation regarding the segment instance reference.
 I only referred to option (1) as the simple first test for valid cache, and then (2). So,
in UUIDDocId, I would create a weakReference to MultiIndexReader and a seperate one to CachingIndexReader
(a single segment) . If (1) is valid, you are ready, otherwise check (2). If (1) is equally
fast as (2) (performance test) I agree on removing (1)

I really think we are on the same track, and are talking about the same mechanism for maintaining
the cache. I was referring to a light weight instance validity  test first, and then fall
back to the segment instance test.  But, as you indicate, this one might be totally useless
in a frequently modifying workspace. 

I'll try to implement some of these options sunday, and try to capture some performance figures.
WDYT?


> UUIDDocId cache does not work properly because of weakReferences in combination with
new instance for combined indexreader 
> ---------------------------------------------------------------------------------------------------------------------------
>
>                 Key: JCR-1213
>                 URL: https://issues.apache.org/jira/browse/JCR-1213
>             Project: Jackrabbit
>          Issue Type: Improvement
>          Components: query
>    Affects Versions: 1.3.3
>            Reporter: Ard Schrijvers
>             Fix For: 1.4
>
>
> Queries that use ChildAxisQuery or DescendantSelfAxisQuery make use of getParent() functions
to know wether the parents are correct and if the result is allowed. The getParent() is called
recursively for every hit, and can become very expensive. Hence, in DocId.UUIDDocId, the parents
are cached. 
> Currently,  docId.UUIDDocId's are cached by having a WeakRefence to the CombinedIndexReader,
but, this CombinedIndexReader is recreated all the time, implying that a gc() is allowed to
remove the 'expensive' cache.
> A much better solution is to not have a weakReference to the CombinedIndexReader, but
to a reference of each indexreader segment. This means, that in getParent(int n) in SearchIndex
the return 
> return id.getDocumentNumber(this) needs to be replaced by return id.getDocumentNumber(subReaders[i]);
and something similar in CachingMultiReader. 
> That is all. Obviously, when a node/property is added/removed/changed, some parts of
the cached DocId.UUIDDocId will be invalid, but mainly small indexes are updated frequently,
which obviously are less expensive to recompute.

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